#assert #test


A proc macro for turning test helper functions into assert! style macros

11 releases (stable)

1.0.7 Sep 22, 2021
1.0.6 Sep 17, 2021
0.2.6 Sep 16, 2021
0.1.5 Sep 15, 2021

#69 in Testing

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MIT license

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Rust crates.io docs.rs



The assert_fn library supplies a proc macro which can be used to turn test helper functions into assert! style macros. It is designed to be used where your test helper is performing an assert for you, for example:

fn check_eq_if_doubled(a: usize, b: usize) {
  assert_eq!(a * 2, b)

check_eq_if_doubled(1, 2);
check_eq_if_doubled(2, 4);
check_eq_if_doubled(4, 8);

There are two reasons you'd want to do this:

  1. Readability - assert! style macros are easy to spot in a test. They help others reading your test understand that this is where you are asserting correctness.
  2. Traceability - In the above example, if you got a failure in one of your calls to check_eq_if_doubled the panic would originate on line 2, rather than the line which triggered the failure. In more complex tests this can make it hard to track down where the test is broken.

Using assert_fn the above can be written as:

use assert_fn::assert_fn;

fn eq_if_doubled(a: usize, b: usize) -> bool {
    a * 2 == b

assert_eq_if_doubled!(1, 2);
assert_eq_if_doubled!(2, 4);
assert_eq_if_doubled!(4, 8);

Or if you want to use assert_eq! instead of assert! so you can see what the values were, just return a tuple instead of a bool:

use assert_fn::assert_fn;

fn eq_if_doubled(a: usize, b: usize) -> (usize, usize) {
    (a * 2, b)

assert_eq_if_doubled!(1, 2);
assert_eq_if_doubled!(2, 4);
assert_eq_if_doubled!(4, 8);

In both of these examples the failure will be logged against the line in your test on which the error originated instead of inside a line inside the eq_if_doubled.

See the Rust docs for lots more examples.


Supported fields on the #[assert_fn] macro.


Only supported on functions returning a tuple, or Result of a tuple.

Provides a default message when the assert fails. The first item in the list must be a string literal. Additional items in the list are destructured from the returned tuple and can be substituted into the message via named properties. Any unused tuple fields prior to one you wish to use must be noted as a _.

Example consuming the third property on a returned tuple.

use assert_fn::assert_fn;

#[assert_fn(message("Nope! But {panic}", _, _, panic))]
fn meaning_of_life(a: usize) -> (usize, usize, String) {
    (a, 42, "don't panic!".to_string())


If the export field is supplied to #[assert_fn] then #[macro_export] is added to your generated macro so it can be used elsewhere.

use assert_fn::assert_fn;

fn is_ten(num: usize) -> (usize, usize) {
    (num, 10)

Trouble Shooting

Because macros have to be defined before they can be used, your helper functions must be declared above the tests you want to use them in.

If you get a cannot find macro error check that your functions are in the correct order.


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